Process of removing foreign matter from magnesium surfaces



Patented Apr. 15, 1947 PROCESS OF REMOVING FOREIGN MATTER FROM MAGNESIUM SURFACES Roy Shawcross, Salina, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a

corporation of Pennsylvania No Drawing. Application May 25, 1943, Serial No. 488,403

2 Claims. (Cl. 252-136) v This invention relates to the treatment of magnesium surfaces to remove foreign matter therefrom. The contamination of magnesium surfaces by foreign materials may result from a number of causes. Such foreign materials are usuall non-metallics included in or on the surface as the result of manufacturing operations, a common example being the carbonaceous deposit formed during the working of magnesium by the lubricant which is necessarily used in such working operations as rolling and drawing. The foreign-material may firmly adhere to the metal surface or it may be imbedded therein. In many instances mere wiping or superficial abrasion of the surface will not remove these materials and a others may be used with good results with the one chemical treatment is necessary. The result of exception that Where iron is present in the soluthe chemical treatment may be to dissolve all or tion theme of ammonium acid sulphate may, as a part of the foreign material and to dissolve a hereinafter described, be disadvantageous if a film of magnesium, thus mechanically releasclean surface without stains is desired. In pracing and eliminating-the foreign material. In any tice the solution as used should contain about 5 event, the use of such chemical treatment is beset per cent to about per cent by weight of one with difficulties, particularly where large quanof these compounds, or if two or more of the titles of magnesium articles are handled. The compounds are present that amount should repchemical treatment if not closely controlled may resent the total present. Less than about 5 per remove too large an amount of the metal from 25 cent by weight of one or a total of these comthe surface, it may etch or attack too deeply thus pounds is not economical because treatment time producing a rough surface, it may leave a dull is prolonged. At higher concentrations than 25 surface of unsatisfactory appearance and it may per cent the reaction is often too violent and unsecondarily stain the surface. To control the atnecessary amounts of the treated metal are distack of the chemical and to produce a clean, solved by the solution. smooth and relatively bright surface, the chem- The process' of obtaining a clean bright magneical treatment is often carried out at low tempersium surface by the use of such solutions is often atures or with comparatively small concentracomplicated by the presence of iron contaminations of the chemical with the result that the time tion in the solution, which contamination may be of treatment is unduly extended. 5 the result of the ordinary iron impurities in com- An object of the invention is to provide a novel mercial grade chemicals, or the result of contam method of chemical treatment by which foreign inationof the solution during operations. In any materials may be satisfactorily removed from event, the iron contamination will, in some cases, magnesium surfaces. Another object is to procause a staining of the sheet. On a bright clean vide a chemical treatment by which this result 40 surface this staining is undesirable although it in may be accomplished in a relatively short time. no Way prevents the removal of foreign matter Among further objects .of the invention is the prowhich is the primary object of the treatment. vision of a method of treatment which will pro- When magnesium sulphate, aluminum sulphate or duce a smooth, clean and relatively bright magan alum are substantial constituents of the treat nesium surface. ing solution, this iron staining seldom, if ever, The invention is directed to the treatment of occurs. But when the acid sulphates of the alkali pure magnesium, magnesium of various grades of metals are used alone, stainin may be quite secommercial purit and magnesium base alloys, vere. This staining may be prevented, I have such as t s containing 70 r cent r r y found, in whole or in substantial part by the adweight of magnesium. The term magnesium 5Q dition of one or more of the following compounds; as used herein and in the appended claims is inhowever, where ammonium acid sulphate is used, tended to define and include all such grades and staining is not always eliminated or diminished: alloys as well as pure magnesium. The soluble sodium polyphosphates-such as In accordance with this invention, the objects (NaPoals, sometimes written NaePeOm and above described are obtained, in whole or in sub- Na6P4O13-in amount of about 0.2- per cent by stantial part, by treatment of the magnesium surface with an aqueous solution containing an acid sulphate of sodium, potassium or ammonium (which elements are herein collectively referred to as, alkali metals since ammonium quite often has the characteristics of such metals) or with an acidified aqueous solution of a-sulphate selected from the group consisting of magnesium sulphate, aluminum sulphate and the double sulphates of alkali metal and aluminum, commonly referred to as alums. These compounds may be used singly or together. Usually I prefer the use of sodium acid sulphate since it gives excellent results and is cheap and plentiful but any of the 3 weight of the total solution. Increasing amounts are not harmful except as operating costs are increased, but generally I prefer to limit the amounts used to less than about per cent by weight.

At least one of the aliphatic acids selected from the group consisting of acetic, tartaric and citric acids and used in total amount of not less than about 0.5 per cent by weight of the solution. Usually not more than about per cent by weight of such an acid, or acids, is desirable because of the unnecessary cost involved in the use of further amounts.

At least one acid selectedfrom the group con-- sisting of tannic acid, gallic acid and pyrogallicacid used in amount of not less than 0.1 per cent nor more than about 5 per cent by weight of the. solution.

Phosphoric acid used in amount of about-2 to about 5 per cent by weight of the total solution.

Solutions of the acid sulphates of alkali metals are sufiiciently acid for the purposes ofthis invention, but solutions containing magnesium sulphate, aluminum sulphate or an alum must be acidified to an extent generally indicated by a ratio, in the treating solution, of one part of the salt to 0.8 part of sulphuric acid. I have found, however, that the sulphuric acid content of the solution may be as high as 15 per cent by Weight. The method of treatment consists in contacting the magnesium surface with the acid sulphate or acidified sulphate solution, removing the surface from contact with the solution and allowing any solution remaining thereon to drain therefrom. If desired, the residual solution may be washed from the magnesium surface. Bestresults seem to be obtained when the solution is maintained at a temperature. of about 60 to about 90 C. Lower temperatures may be maintained during operation with the result that the cleaning action proceeds at a slower rate. In many cases higher temperatures may result in too strong an attack and the dissolving of unnecessary amounts of metal from the surface undergoing treatment. The exact temperature to be used with any solution is readily determined in accordance with these principles by simple experiment. Similar principles may be applied to the time of immersion necessary for the best results Usually 30 seconds to 3 minutes will give satisfactory commercial results.

The acid sulphates of the alkali metals, magnesium sulphate, aluminum sulphate or an alum may be used singly orin mixture. In the com.- mercial operation of the process the solution is used over and over again and in time the. reaction of the solution with the. magnesiumsurfaces which are being treated will build up in the solution a content of magnesium. sulphate. Also, under repeated reuse thereactants of the original solution are diminished by drag out as the magnesium articles are removed from the solution and by the reaction with the magnesium surface during treatment. However, this introduction into the treating. solution of magnesium sulphate during continued. use of the solution. is a factor of which advantage maybe taken. For instance, if one starts with a solution of sodium acid sulphate, the solution will, after repeated use, lose a material part of its sodium acid sulphate content, but at the same timeit will gain in magnesium sulphate content until eventually the solution is composed largely of magnesium sulphate and sodium acid sulphate is no longer present. in amounts necessary tothe reaction.

' In such a case it is possible to continue the use of the solution if, in accordance with this invention, the solution is from time to time fortified with sulphuric acid to maintain a ratio of free (i. e. uncombined) acid to magnesium sulphate as above mentioned or anexcess ofacid over that ratio. A substance such as one of the polyphosphates may have been used in the original acid sulphate solution to suppress the iron, but

as the magnesium sulphate content of the solution builds up, the future use of the substance is no longer necessary since, as above described, iron staining rarely occurs in the presence of acidified magnesium sulphate.

It ispossiblc, therefore, in the practice of the invention, to start with any one of the acid sulphates mentioned or with acidified solutions of aluminum sulphate or an alum and after the solution has been in use for some time to continue to practice theinvention, without discarding the solution, by acidifying the solution to achieve the proper balance between the acid and. the magnesium sulphate. content acquired by the solution during treatment. Alternatively the solution. may merely be discarded as the reaction and drag out diminish its reactive qualities; Again alternatively, the startin solution may be an acidified solution of magnesium sulphate, in which case after use it may be discarded or the acid balance between acid and magnesium sulphate maintained by the addition of sulphuric acid as the original. acid content and magnesium sulphate content diminishes and the. solution acquiresadditional quantities of magnesium sulphate as; the

l result of the operations.

By the use of the methods of treatment above described, magnesium surfaces may be quickly and efficiently cleaned of foreign matter and a relatively smooth bright appearance achieved. The methods of this invention produce better results in commercial operations than those usually obtained by the useof acids as a cleansing medium.

As an example illustrative of the operation of the invention may be cited an instance where carbonized roll. coating was. removed from magnesium sheet. The sheet was treated for about 2 minutes in asolution containing, 10 per cent by weight of NaI-ISO4 to which had been added 0.2 per cent by weight of (NaPOs) s. The solution was maintained at a temperature of about 158 F. Thereafter the sheet was removed from the solution and all-owed to drain free of solution remaining on its surface. The result was a complete removal of the rollcoating leaving a clean bright surface.

I' claim:

l. The process of removing from a magnesium surface, rollcoatings and similar foreign materials wlii-ch comprises contacting thesurf'ace with an aqueous solution containing from about 5 to about 25 per cent. byweight of an acid sulphate of an alkali. metal and from: about 0.2 to about 15- per centby weight of a soluble sodium poly-- phosphate.

2. The. process of. removing foreign materials from magnesium surfaces which comprises contacting thesurface with anaqueous solution containing in total amount from about 5 to about 25 per cent by weight of at least oneof the sulphates selected from the. group consisting of magnesium sulphate, aluminum: sulphate and double: sulphate of aluminum and. alkali. metal and also contain.- ing sulphuric acid-in amountat least approximating a ratio of one part of. said sulphate to 0.8 part of said acid but not exceeding 15 per cent by Number Name Date weight of said acid. 2,092,913 Fiske Sept. 14, 1937 ROY SHAWCROSS. 1,795,473 Burdick Mar. 10, 1931 2,314,369 Reed Mar. 23, 1943 Th f n IREFEfENCES d 1 th 5 FOREIGN PATENTS e o ow ng re erences are 0 recor n e file of this patent: Number Country Date 500,009 British Feb. 1, 1939 UNITED STATES PA S OTHER REFERENCES Number Name Date 1 Condensed Chemical Dictionary, 3rd ed., re-

9 5 Hey July 18, 1933 vised (1942) Reinhold Pub. Co., N. Y., page 586. 2,287,050 Miller June 23, 1942 (Copy in 54 2,302,939 De Long Nov. 24, 1942 

